1. Field
The present system teaches a new priority system for asynchronous transfer mode, and an application of that new system to a loop back operation.
2. Background
Asynchronous transfer mode or ATM is a telecommunications protocol that allows packet based transfer of information. Cells of information are sent across an information network defined by a number of nodes. The information is sent from node-to-node.
An ATM transport network (i.e., a communication network, which transmits information using ATM cell packets) is known to include an ATM layer and a physical layer. The ATM layer is based on the virtual path/virtual channel (VP/VC) concept. The VC identifies a unidirectional communication capability through which ATM cells are transported. One or more virtual channels (VCs) can be used in a particular virtual path (VP), which also identifies another level of the communication capability through which the ATM cells are transported.
An ATM cell is the smallest information unit. It includes a header field of 5 bytes or octets, and a payload field of 48 bytes or octets. The header field includes VP and VC identifiers. These identifiers are used for routing the information to an intended destination.
Communication in known ATM networks is initiated during a connection setup, after which cells belonging to one connection follow a predetermined path defined by the VPI and VCI on a particular link. The connection control information transferred during setup utilizes a unique Signaling VC (SVC) which is included in the VP. The SVC is identified by the virtual path ID (VPI) and virtual channel ID (VCI).
Cells destined for many different end points are sent over a single physical communications circuit. The header of each cell includes a channel identifier, which is used to control the routing of the cell through the ATM system. The channel identifier determines routing of the cell.
In a typical ATM system there are 256 possible VPIs and 65,536 possible VCIs; thus, there are 16,777,216 possible channel identifiers (VPI/VCIs). One of the many challenges in designing an ATM network is how to handle this huge number of corrections.
Specified traffic control protocols are used to determine the routing of the information. The routing is controlled using conventional addressing techniques.
Further details of ATM are well known in the art. In addition, different flavors and sub-types of ATM are known, including digital subscriber line (xe2x80x9cDSLxe2x80x9d), asymmetric digital subscriber line (xe2x80x9cADSLxe2x80x9d), and other flavors of digital subscriber line (xe2x80x9cXDSLxe2x80x9d).
In all of these communication modes, a message is broken into multiple portions or cells. A conventional ATM system breaks the total message to be sent over ATM into 48 byte data portions. A typical data message might be, for example, 1500 bytes in length. Hence, the 1500 byte message is divided up into 31 of the 48 byte cells.
Specified traffic control protocols are used to determine the routing of the information. The routing is controlled using conventional addressing techniques.
ATM has the ability to provide a guaranteed quality of service. This compares with other information services, which have no such guarantee. Data packages are subdivided into classes of services. The ATM switches prioritize each data packet routing based on the quality of service.
Conventional quality of service classes include substantially constant bit rate (xe2x80x9cCBRxe2x80x9d), variable bit rate (xe2x80x9cVBRxe2x80x9d) and unspecified bit rate (xe2x80x9cUBRxe2x80x9d). These classes are based on ITU recommendation 1.371 entitled Traffic Control, congestion control in BSIDN and on version 4.0 of xe2x80x9cthe Traffic Management specificationxe2x80x9d. Connection data transmission rates may be set on a semi-permanent basis depending the requirements for the connection.
One very good quality of service is CBR or constant bit rate. The data is guaranteed to arrive at a constant rate. The ATM switches along the virtual path prioritize the data packages. The CBR traffic gets priority over the remaining traffic. Therefore, even if there is one or more congested switches, the CBR still gets through quickly.
UBR, or unspecified bit rate is at the other end of the scale as compared with CBR. There is no guarantee about when the data will arrive at the destination.
Other classes of service have been defined by the ATM forum. However, these can require extensive and complicated handling by the ATM switch. Examples include AVR (available variable rate) or VVR (variable variable rate). Each requires that the ATM switch be significantly reconfigured to handle the new requirements.
The present specification defines another quality of service class for such a communication connection. The present system uses the relatively simple interface of CBR/UBR to provide an intermediate form of service using the same memory infrastructure as in CBR/UBR. In particular, the present system defines a class of service that accommodates loop back within an ATM network. This intermediate form of service enables an unspecified bit rate, which can provide protection against backlog in a congested switch, without giving of priority over all other parts.